The invention is directed to a laser radiation source, preferably for processing materials, as well as to an arrangement for processing material comprising a laser radiation source and to the operation thereof.
When processing materials with focused energy beams such as, for example, electron beams or laser beams, there are applications wherein structures must be produced that make high demands of the focused energy beam with respect of its beam geometry and the focusability of the beam. At the same time, however, a high beam power is required.
A typical case wherein extremely fine structures must be produced on a processing surface is the production of printing forms, whether for rotogravure, offset printing, letter press printing, silk screening or flexo-printing or for other printing processes. In the production of printing forms, it is necessary to produce extremely fine structures on the surface of the printing forms, since highly resolved image information such as text, screened images, graphics and line work must be reproduced with the surface of the printing forms.
In rotogravure, the printing forms were produced in the past with etching, which had led to good results; the etching, however, was replaced over the course of time by more environmentally friendly engraving with electromagnetically driven diamond styli. Printing cylinders whose surface is composed of copper are normally employed as printing forms in rotogravure, these fine structures required for the printing being engraved thereinto in the form of cups with the diamond stylus. The printing cylinders are introduced into a printing press after they are produced, the cups being filled with ink therein. Subsequently, the excess ink is removed with a doctor blade and the remaining ink is transferred onto the printed matter during the printing process. Copper cylinders are thereby employed because of their long service life in the printing process. A long service life is required given large editions, for example, in particular, in magazine printing or packaging printing, since the surface of the printing form wears in the printing process as a result of the influence of the doctor blade and of the printed matter. In order to extend the service life even further, the printing cylinders are provided with a copper layer that has been galvanized on; on the other hand, solid cylinders of copper are employed. Another possibility of making the service life even longer is comprised in galvanically chrome plating the copper surface after the engraving. In order to achieve an even longer service life, what is referred to as “hot chrome plating” is additionally applied, whereby the galvanic process is carried out under elevated temperature. The longest service lives that could previously be obtained were achieved therewith. Deriving therefrom is that copper is the most suitable as the material for the surface of rotogravure cylinders. Materials other than copper have not hitherto proven themselves for large editions.
When producing the cups, the drive of the diamond stylus occurs via an electromechanically driven magnet system having an oscillating armature to which the diamond stylus is secured. Such an electromechanical oscillatory system cannot be made arbitrarily fast because of the forces that must be exerted in order to engrave the cups. This magnet system is therefore operated above its resonant frequency so that the highest engraving frequency, i.e. the highest engraving speed can be achieved. In order to increase the engraving speed even further, a number of such engraving systems have been arranged side-by-side in the axial direction of the copper cylinder in given current engraving machines. This, however, still does not suffice for the short engraving time of the printing cylinders required currently, since the engraving time directly influences the actuality of the printing result. For this reason, rotogravure is not employed for newspaper printing but mainly for magazine printing.
Upon utilization of a plurality of engraving systems, a plurality of what are referred to as lanes are simultaneously engraved into the surface of the printing cylinder. For example, such a lane contains one or more entire magazine pages. One problem that thereby arises is that cups having different volumes are generated in the individual lanes given the same tone value to be engraved, this occurring because of the different engraving systems that are driven independently of one another and leading to differences in the individual lanes that the eye detects during later observation. For this reason, for example in packaging printing, only one engraving system is employed so that these errors, which are tolerated in magazine printing, do not occur.
When engraving the cups, the cup volume is varied dependent on the image content of the master to be printed. The respective tone value of the master should thereby be reproduced as exactly as possible during printing. When scanning the masters, the analog-to-digital converters having, for example, a resolution of 12 bits are utilized for recognizing the tone value gradations for reasons of image signal processing (for example, gradation settings), this corresponding to a resolution of 4096 tone values in this case. The signal for the drive of the electromagnetic engraving system is acquired from this high-resolution image information, said signal usually being an 8-bit signal corresponding to a resolution into 256 tone value gradations. In order to generate the corresponding volumes that are required for achieving this scope of gradations, the penetration depth of the diamond stylus into the copper surface is varied with the drive of the magnet system, whereby the geometry of the cups changes between approximately 120 μm diameter given a depth of 40 μm and approximately 30 μm diameter given a depth of 3 μm. Because only an extremely small range of variation in the depth of the cups between 40 μm and 3 μm is available, the penetration depth of the stylus with which the cups are engraved must be exactly driven to fractions of a μm in order to reproducibly achieve the desired range of gradation. As can be seen therefrom, an extremely high precision is required in the engraving of the cups, at least as regard to the generation of the required diameters and depths of the cups. Since the geometry of the engraved cups is directly dependent on the shape of the stylus, extremely high demands are also made of the geometry of the diamond stylus which, as has been shown, can only be achieved with extremely high expense and with a high rejection rate in the manufacture of the styli. Moreover, the diamond stylus is subject to wear since, when engraving a large printing cylinder having fourteen lanes, a circumference of 1.8 m and a length of 3.6 m given a screen of 70 lines/cm—which corresponds to a plurality of 4900 cups/cm2, a stylus must engrave approximately 20 million cups. When one of the diamond styli breaks off during the engraving of a printing cylinder, then the entire printing cylinder is unuseable. On the one hand, this causes a considerable financial loss and, on the other hand, represents a serious loss of time since a new cylinder must be engraved, postponing the start of printing by hours. For this reason, users frequently replace styli earlier than necessary. As can also be seen therefrom, the endurance of the diamond styli is also a critical concern.
All in all, electromagnetic engraving is well-suited for producing high-quality rotogravure cylinders; however, it has a number of weak points and is extremely complicated and one would like to eliminate these disadvantages with a different method.
The cups produced in this way, which are intended to accept the ink later, are also arranged on the surface of the printing form in conformity with a fine, regular screen, namely the printing screen, whereby a separate printing cylinder is produced for each ink, and whereby a different screen having a different angle and different screen width is respectively employed. When printing in the printing press, given these screens, narrow bridges remain between the individual cups, these supporting the doctor blade that removes the excess ink after the inking. Another disadvantage of this operating mode of this electromechanical engraving is that texts and lines must also be reproduced in screened fashion, which leads to step-patterns in the contours of the written characters and the lines that the eye perceives as being disturbing. This is one disadvantage compared to the widespread offset printing wherein this stepping can be kept an order of magnitude lower, which can then no longer be perceived by the eye, and which leads to a better quality that rotogravure could hitherto not achieve. This is a serious disadvantage of the rotogravure process.
In rotogravure, no stochastic screens can be generated wherein the size of the cups and the position of the cups can be randomly distributed corresponding to the tone value; this is not possible when engraving with the diamond stylus. Such stochastic screens are also frequently referred to as “frequency-modulated screens” that have the advantage that details can be reproduced far better with no Moirè, this also leading to a better image quality than in rotogravure.
It is also known to utilize the electron beam engraving method applied in the processing of materials for generating the cups, this having exhibited extremely good results because of the high energy of the electron beam and the incredible precision with respect to the beam deflection and beam geometry.
This method is described in the publication, “Schnelles Elektronenstrahlgravierverfahren zur Gravur von Metallzylindem”, Optik 77, No. 2 (1987) pages 83-92, Wissenschaftliche Verlagsgesellschaft mbH Stuttgart. Due to the extremely high expense that is required for the hardware and electronics, electron beam engraving has hitherto not prevailed in practice for the engraving of copper cylinders for rotogravure but only in the steel industry for surface engraving of what are referred to as textured drums for sheet metal manufacture wherein textures are rolled into the sheets.
It has been repeatedly proposed in the trade literature as well as in the patent literature to engrave copper cylinders with lasers. Since copper, however, is an extremely good reflector for laser radiation, extremely high powers and, in particular, extremely high power densities of the lasers to be employed are required in order to penetrate into the copper and melt it. There has hitherto not been any laser engraving unit with laser radiation sources having a correspondingly high power density and energy with which one succeeds in providing the copper cylinders for rotogravure with the required cup structure in the copper surface.
Attempts have nonetheless been made to utilize lasers for rotogravure in that a switch has been made to materials other than copper. Thus, for example, the publication DE-A-19 20 323 has proposed to prepare copper cylinders with chemical etching such that the surface of the copper cylinder already comprises cups that have a volume that corresponds to the maximum printing density. These cups are filled with a solid filler material, for example plastic. Much of the filler material is then removed with a laser until the desired cup volume has been achieved. This method in fact manages with a lower laser power than would be necessary in order to melt and evaporate the copper as in electron beam engraving. In this method, however, the remaining plastic is attacked by the solvent of the ink in the printing process and is decomposed, so that only a low print run is possible. This method has not proven itself in practice and has thus not been utilized.
The publication of the VDD Seminar Series, “Direktes Lasergravierverfahren für metallbeschichtete Tiefdruckzylinder”, published within the framework of a “Kolloquium vom Verein Deutscher Druckingenieure e.V. und dem Fachgebiet Druckmaschinen und Druckverfahren, Fachbereich Maschinenbau, Technische Hochschule Darmstadt”, by Dr. phil. Nat. Jakob Frauchiger, M D C Max Dätwyler, A G, Darmstadt, 12 Dec. 1996, has proposed that rotogravure cylinders plated with zinc be engraved by a quality-switched Nd:YAG high-power solid-state laser pumped with arc lamps. In this method, the volume of the cups is defined by the optical power of the laser. The laser power required for the engraving is transmitted onto the cylinder surface via an optical fiber whose output is imaged onto the cylinder surface through a variable focusing optics. One disadvantage of this method is that the arc lamps required for pumping the laser have a relatively short service life and must be replaced after approximately 500 hours of operation. The engraving cylinder becomes unuseable given a failure of the pump light source during the engraving. This corresponds to a failure of the diamond stylus in electromechanical engraving and results in the same disadvantages. A preventative replacement of the arc lamps is cost-intensive and work-intensive, particularly since one must count on the fact that the laser beam must be readjusted in position after the replacement of the lamps. These lamp-pumped solid-state lasers also have a very poor efficiency since the laser-active material absorbs only a slight fraction of the available energy from the pump source, i.e. from the arc lamp here, and converts into laser light. Particularly given high laser powers, this means a high electrical connection cost, high operating costs for electrical energy and cooling and, in particular, a considerable expense for structural measures due to the size of the laser and the cooling unit. The space requirements are so high that the laser unit must be located outside the machine for space reasons, this in turn being accompanied by problems in bringing the laser output onto the surface of the printing cylinder.
A critical disadvantage of this method is that zinc is significantly softer than copper and is not suitable as a surface material for printing cylinders. Since the doctor blade with which the excess ink is removed before printing in the printing press is a steel blade, the zinc surface is damaged after a certain time and the printing cylinder becomes unuseable. A printing cylinder having a surface of zinc therefore does not even begin to approach as long a service life in printing as a printing cylinder having a surface of copper. Printing forms having a zinc surface are therefore not suitable for high press runs.
Even if the zinc surface is chrome-plated after the engraving, as has been also proposed in order to lengthen the service life, the durability does not come close to that of normal copper cylinders. Chrome does not adhere to zinc as well as it adheres to copper and what is referred to as “hot chrome plating”, which is successfully employed given copper cylinders in order to achieve an optimum adhesion of the chromium on the copper, is not possible given zinc since the zinc would thereby melt. Since the chrome layer does not adhere very well on the zinc, it is likewise attacked by the doctor blade, which leads to a relatively early failure of the printing cylinders. When, in contrast thereto, copper cylinders are chrome-plated according to this method, then incredibly high press runs are possible since the chromium firmly adheres on the copper surface, so that these copper cylinders out perform the chrome-plate zinc cylinders by far.
It proceeds from the publication EP-B-0 473 973, which is likewise directed to the method described above, that an energy of 6 mWsec is required in this method given zinc for cutting a cup having a diameter of 120 μm and a depth of 30 μm. An energy of 165 mWsec is recited in this publication for copper, this amounting to a factor of 27.5 for the required laser power. Lasers having a continuous-wave performance of several kilowatts given good beam quality are thus required in order to produce cups in copper with a speed that is accessible for the printing industry. Such a power, however, cannot be produced with the laser arrangement described above. For this reason, it is likewise only possible to engrave a zinc surface.
Such a laser arrangement, which is composed of a single solid-state laser, in fact makes it possible to process rotogravure cylinders having a zinc surface; if, however, one wishes to utilize the advantages of the copper surface and stay with copper cylinders and engrave these with a laser, the high power density required for penetration into the surface of the copper and the high energy required for melting the copper must be inevitably exerted. This, however, has not hitherto been successfully done with a solid-state laser.
It is known that the beam quality in solid-state lasers, i.e. the focusability, decreases with increasing power. Even if the power of the solid-state lasers were to be driven up or if a plurality of solid-state lasers were directed onto the same cup or parts thereof, it would therefore not be possible to satisfactorily engrave copper cylinders for rotogravure with such a laser because the precision of the laser beam, as offered by the electron beam, required for generating the fine structures cannot be achieved. If the laser power were increased given this apparatus, then a further problem would arise: the focusing of high radiant intensity into optical fibers is, as known, difficult. The fibers burn at high power as a consequence of misadjustment at the infeed location. If one wishes to avoid this, however, the fiber diameter would have to be enlarged which, however, in turn has the disadvantage that the fiber diameter would have to be imaged onto the processing material with even greater demagnification. A demagnified imaging, however, leads to an increase in the numerical aperture on the processing surface and, consequently, to a reduced depth of field on the processing surface. As proposed, the distance from the processing surface could be kept constant. When, however, the beam penetrates into the surface of the material, then a defocusing automatically derives. This has a disadvantageous influence on the required power density and on the exact dot size. Since, however, the diameter of the processing spot and the energy of the beam determine the size of the cup, it then becomes difficult to make the cup size as exactly as required by the desired tone value. For this purpose, it would also be necessary that the laser power is exactly constant and also remains constant over the entire time that is required for a cylinder engraving. When this is not the case, the cup size changes and the cylinder becomes unuseable. This cannot be compensated by varying the size of the processing spot since it is not possible to adequately vary the processing spot in shape.
Further, a complicated modulator is required given such an arrangement. As known, modulators for extremely high laser powers are slow, this leading to a reduction of the modulation frequency and, thus, of the engraving frequency. When, however, the engraving frequency is too low, the energy diffuses into the environment of the processing spot on the processing surface without cutting out a cup. It is therefore necessary to also exert a high power in addition to the high energy for the cutting.
The publication “Der Laser in der Druckindustrie”, by Werner Hülsbusch, page 540, Verlag W. Hülsbusch, Konstanz, describes that it is particularly a matter of a high power density in processing materials. Given power densities of typically above 107 through 108 W/cm2, a spontaneous evaporation of the material occurs in all materials, this being accompanied by a sudden absorption rise, which is especially advantageous since the laser power is then no longer reflected from the metal surface. When, for example, a laser source of 100 W is available, then the processing spot diameter may not be larger than 10 μm in order to arrive at these values in the region, as proceeds from the following equation: 100 W:(0.001 cm×0.001 cm)=108 W/cm2.